Current hammer mills used in making inert materials have several problems: an elevated percentage of recycling with repercussions on productivity, a high percentage of dust in the obtained product, problems of environmental impact and guardianship of the health of the workers in the environments of the job due to the the enormous quantity of dust that is emitted by the mill, bad granulometry and polyhedric nature of the obtained product (including a scarce presence of thin parts with an excess of dust), and strong usury of the hammers and the armored walls.
A principal cause of ineffectiveness is the strong peripheral speed of the hammers necessary to break the inactive material, which does not allow an easy passage of the same material in front of the hammers.
In tertiary mills, to be able to arrive to treat chippings up to thirty millimeters in diameter, is may be necessary to arrive to a peripheral speed around 70 m/s, while in the secondary mills, with chippings of the order of a hundred millimeters at the most, it is necessary to reach almost 40 m/s. Such speeds are too high to allow the penetration of the inert material with those dimensions, in an extremely little time (around 3/100 of second for the tertiary mill and around double for the secondary mill), on the front of the hammer.
The physical phenomenon that may be verified is clearly seen if a simulation of the operation of a mill is performed, for example of a tertiary mill, to two hammers for the production of sand, with a personal computer. As soon as chippings are introduced in the mill it is noticed that the hammer, already from the first impact, and having once taken a certain quantity of inert materials, interferes with chipping particles. It is created, that is, an interference phenomenon between the superior particles not intercepted, whose entity is strongly tied, over that to the aforesaid peripheral speed of rotation (above all), also to the thickness of heading of the hammer and the thickness of the chippings flow of feeding that arrives from the height in free fall. These particles of chippings, do not regularize because of the high frequency of beat of the hammer in time (about thirty times a second for the tertiary mills to two hammers). And then, above all when the edge of the hammer begins to become round for the usury, the chippings, practically do not arrive anymore to be intercepted by the front of the hammer, as they float around the perimetric circumference of the rotor and the wall armored of the mill, through a space that becomes more and more tightened, thin to be forced to the crushing in a point that depends on the dimensions of the same chippings.
From here the necessity to endow the traditional mill of a special register of approach of the armored wall, in comparison to the perimetric circumference of the hammer (to establish the maximum sizing of the inert material from obtaining to crushing and to compensate the usury of the superior part of the hammer).
The results of this traditional system to crushing are, inevitably, negative.